Electric-resistance-welded steel pipe with high strength

ABSTRACT

An electric-resistance-welded steel pipe with high strength consists essentially of 0.05 to 0.30 wt. % C, 0.6 to 2.0 wt. % Mn, 0.03 to 0.70 wt. % Si, and 0.01 to 0.05 wt. % Nb. The balance is Fe and inevitable impurities, the steel pipe having a martensitic phase ratio of 60 to 100% and having a tensile strength of 120 to 170 kgf/mm2.

This application is a division of application Ser. No. 07/813,361 filedDec. 24, 1991, now U.S. Pat. No. 5,286,309 issued Feb. 15,1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric-resistance-welded steelpipe with high strength for a door impact bar used for cars, and moreparticularly to an electric-resistance-welded steel pipe withhigh-strength for an impact bar used for the inside of an automobile.

1. Description of the Related Arts

In regard to doors for vehicles such as automobiles, a reinforcingmaterial made of steel or the like is applied to the inside of a door tosecure the safety of passengers in a car when another car crashes intothe side of the car and to lighten the car weight. This reinforcementmaterial is referred to as a door impact bar. Japanese PatentApplication Laid Open No.197525/90 discloses a reinforcement material ofa tube or a reinforcement material having a tensile strength of from 60to 100 kgf/mm² after stress relief annealing at a low temperature.

It is undesirable, however, to use the prior art reinforcement materialas the door impact bar. The reinforcement material having a tensilestrength of from 60 to 100 kgf/mm² is insufficient for the door impactbar. Even though only this tensile strength is improved, propertieseffective for the door impact bar cannot be obtained.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide anelectric-resistance-welded steel pipe with high strength which isdesirable as a door impact bar.

To attain the above-described object, the present invention provides anelectric-resistance-welded steel pipe with high strength, consistingessentially of 0.05 to 0.30 wt. % C, 0.6 to 2.0 wt. % Mn, 0.03 to 0.70wt. % Si, the balance being Fe and inevitable impurities, said steelpipe having a martensitic phase ratio of 60 to 100% and having a tensilestrength of 120 to 170 kgf/mm².

Further, the present invention provides an electric-resistance-weldedsteel pipe with high strength, consisting essentially of 0.05 to 0.30wt. % C, 0.6 to 2.0 wt. % Mn, 0.03 to 0.70 wt. % Si, at least oneselected from the group consisting of 0.05 wt. % Nb or less, 1.2 wt. %Cr or less, 0.5 wt. % Mo or less, 0.1 wt. % V or less, 0.05 wt. % Ti orless and 0.005 wt. % B or less, the balance being Fe and inevitableimpurituies, said steel pipe having a martensitic phase ratio of 60 to100% and having a tensile strength of 120 to 170 kgf/mm².

The above objects and other objects and advantages of the presentinvention will become apparent from the following detailed description,taken in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an schematic illustration describing a three-point bendingtest wherein properties of an impact bar made of anelectric-resistance-welded steel pipe of the present invention weretested, and

FIG. 2 is an graphical presentation showing a distribution of hardnessesof an impact bar adjacent to a seam welded portion of the impact barmade of the electric-resistance-welded steel pipe of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The reason for limiting the composition of the steel will now bedescribed as follows:

C is an element which produces a martensite structure and increasinghardness of the martensite. C is an element indispensable for obtaininga strength which is an object of the present invention. When the contentof C is less than 0.05 wt. %, a predetermined strength cannot beobtained. When the content of C exceeds 0.30 wt. %, a slab is liable tobe cracked, the impact resistance of a final product is lowered or theweldability of the final product in CO₂ gas shielded arc welding islowered. The content of 0.05 to 0.30 wt. % C is desirable.

Mn is an element for increasing the hardenability of austenite. Thecontent of 0.6 wt. % Mn is required to control an amount of martensiteand to obtain a predetermined strength of a slab. When the content of Mnexceeds 2.0 wt. %, an effect is saturated in the case of carrying out acontinuos annealing of steel by means of a rapid water quenching.Conversely, such content has a an adverse bad influence on a bendingworkability during manufacturing of a pipe. The content of 0.6 to 2.0wt. % Mn is desirable.

Si is a deoxidizing element. Si contributes to an increase of mechanicalproperties of a steel sheet and increases the ductility of the steelsheet. When the content of Si is less than 0.03 wt. %, predeterminedmechanical properties or ductility cannot be obtained. However, when thecontent of Si exceeds 0.7 wt. %, spattering is often generated duringelectric-resistance welding, which lowers mechanical properties and theimpact resistance of an electric-resistance-welded pipe. The content of0.03 to 0.70 wt. % Si is desirable.

Cr, Mn and Si are elements indispensable for anelectric-resistance-welded steel pipe of the present invention, and thebalance elements are Fe and impurities. The quality of anelectric-resistance-welded portion of a pipe is improved by theabove-mentioned range of elements.

In the present invention, however, at least one element selected fromthe group consisting of Nb, Cr, Mo, V, Ti and B can be added to steelwithin a predetermined range as the case may be required. The propertiesof each of the elements are effectively utilized for purposes.

Nb contributes to making a finer martensite structure of steel and toimproving the hardenability of steel. Cr, Mo, V, Ti and B are elementsfor increasing the hardenability. When the content of Nb exceeds 0.05wt. %, the spatterring is liable to be generated duringelectric-resistance welding, which lowers the mechanical properties ofan electric-resistance welded portion. 0.05 wt. % is determined to bethe upper limit of the content of Nb. When the content of Cr exceeds 1.2wt. %, the content of Mo exceeds 0.5 wt. %, the content of V exceeds 0.1wt. % or the content of Ti exceeds 0.05 wt. %, oxides such as Cr₂ O₃,MoO₂, V₂ O₃ and TiO₂ reside in the electric-resistance welded portion,which deteriorates the mechanical proporitics of the electric-resistancewelded portion or decreases the impact resistance of a steel pipe. Whenthe content of B exceeds 0.005 wt. %, workability of steel deteriorates.

The martensitic phase ratio is desired to be from 60 to 100% When themartensitic phase ratio is less than 60%, the tensile strength of steelis rapidly decreased. The tensile strength cannot be increased to 120kgf/mm² or more.

Example

The present inventors produced a steel pipe having a wall thickness of2.1 mm and a diameter of 34.0 mm, which has elements and structures asshown in Table 1 (A) and (B). Nos. 1 to 14, Nos. 17 and 18 show Examplesof the present invention. Nos. 15, 16 and 19 show Comparisons.

A slab having composition as shown in Table 1 (A) was used. Nos. 1 to 4have the composition of 0.05 to 0.30 wt. % C, 0.6 to 2.0 wt. % Mn, 0.03to 0.70 wt. % Si, the balance being Fe and inevitable impurities.

Nos. 5 to 9, 14, 17 and 18 have the composition of 0.05 to 0.30 wt. % C,0.6 to 2.0 wt. % Mn, 0.03 to 0.70 wt. % Si, one element selected fromthe group consisting of 0.05 wt. % Nb or less, 1.2 wt. % Cr or less, 0.5wt. % Mo or less, 0.1 wt. % V or less, 0.05 wt. % Ti or less and 0.005wt. % B or less, the balance being Fe and inevitable impurituies.

No. 10 has the composition of 0.05 to 0.30 wt. % C, 0.6 to 2.0 wt. % Mn,0.03 to 0.70 wt. % Si, 1.2 wt. % Cr or less and 0.5 wt. % Mo or less.

No. 11 has the composition of 0.05 to 0.30 wt. % C, 0.6 to 2.0 wt. % Mn,0.03 to 0.70 wt. % Si, 0.05 wt. % Ti or less and 0.005 wt. % B or less.

No. 12 has the composition of 0.05 to 0.30 wt. % C, 0.6 to 2.0 wt. % Mn,0.03 to 0.70 wt. % Si, 1.2 wt. % Cr or less, 0.5 wt. % Mo or less and0.1 wt. % V or less.

No. 13 has the composition of 0.05 to 0.30 wt. % C, 0.6 to 2.0 wt. % Mn,0.03 to 0.70 wt. % Si, 0.05 wt. % Nb or less, 1.2 wt. % Cr or less, 0.5wt. % Mo or less, 0.1 wt. % V or less, 0.05 wt. % Ti or less and 0.005wt. % B.

A steel sheet produced by hot-rolling said slab or a steel sheetproduced by hot-rolling and cold-rolling said slab was continuouslyannealed. Treatments such as soaking in the continuous annealing,continuous water quenching and heat treatment were carried out under thefollowing conditions:

The soaking treatment was carried out at a temperature of 870°C. Thetemperature within a range of from 800° to 900° C. was appropriatelyused. The water quenching was carried out at a line speed of 50 to 120m/min and at a cooling rate of 800° to 1500° C. /sec. The heat treatmentwas carried out at a temperature of 100° to 300° C. for ten minutes.

Mechanical properties of the manufactured steel pipe are shown in Table1 (B). The steel pipe according to the Example of the present inventionhad a high strength of 122 to 168 kgf/mm². The yield point of the steelpipe was 85 to 151 kgf/mm². The maximum load at the time of thethree-point bending test was 1150 kgf or more. Except for No. 18, themaximum load was 1200 kgf or more. The maximum load at the time of thethree-point bending test according to the Comparison was 1100 kgf orless. As a result, it was confirmed that the steel pipe according to thepresent invention had desirable properties for an impact bar for doorsof automobiles. A method for measuring the maximum load at the time ofthe three-point bending test is shown in FIG. 1. A specimen 1 was put ona bending span of 822 mm. A bending jig 2 was pressed on the midpoint ofthe specimen 1.

A distribution of hardnesses of steel of the present invention adjacentto the electric-resistance welded portion of the steel pipe is shown inFIG. 2. Although the hardnesses of steel adjacent to theelectric-resistance welded portion of the steel pipe were somewhatdecreased as shown in FIG. 2, a portion having a high hardness wasformed again at the center of the electric-resistance welded portion. Itis clearly seen from a high tensile strength and high maximum load thata formation of the portion having a high hardness at the center of theelectric-resistance welded portion realizes properties desirable as animpact bar.

Even in the case where the water quenching and annealing are applied toan electric-resistance welded steel pipe produced by using a steel ofthe present invention, an electric-resistance welded steel pipe with ahigh strength for the impact bar having a tensile strength of 120 to 170kgf/mm² can be obtained.

According to the present invention as described above, anelectric-resistance welded steel pipe having a high strength and otherproperties desirable for a impact bar for doors of automobiles and othervehicles to safety secure passengers in a car can be obtained, whichproduces an industrially great effect.

                                      TABLE 1 (A)                                 __________________________________________________________________________    Chemical Composition (wt. %)                                                  No C  Si Mn Nb Cr Mo V  Ti  B  Remarks                                        __________________________________________________________________________     1 0.20                                                                             0.33                                                                             0.80                  Example                                         2 0.15                                                                             0.33                                                                             1.76                  Example                                         3 0.20                                                                             0.40                                                                             1.76                  Example                                         4 0.27                                                                             0.33                                                                             0.80                  Example                                         5 0.20                                                                             0.40                                                                             1.76                                                                             0.015              Example                                         6 0.15                                                                             0.33                                                                             1.60  1.0             Example                                         7 0.15                                                                             0.33                                                                             1.60     0.3          Example                                         8 0.15                                                                             0.33                                                                             1.60        0.060     Example                                         9 0.15                                                                             0.33                                                                             1.60           0.020  Example                                        10 0.15                                                                             0.33                                                                             1.60  1.1                                                                              0.4          Example                                        11 0.15                                                                             0.33                                                                             1.60           0.010                                                                             0.001                                                                            Example                                        12 0.15                                                                             0.33                                                                             1.60  0.6                                                                              0. 2                                                                             0.042     Example                                        13 0.07                                                                             0.10                                                                             0.65                                                                             0.005                                                                            0.4                                                                              0.3                                                                              0.044                                                                            0.008                                                                             0.001                                                                            Example                                        14 0.15                                                                             0.33                                                                             1.60                                                                             0.010              Example                                        15 0.03                                                                             0.10                                                                             0.80                                                                             0.005                                                                            0.4                                                                              0.3                                                                              0.044                                                                            0.008                                                                             0.001                                                                            Comparison                                     16 0.07                                                                             0.10                                                                             0.55                                                                             0.005                                                                            0.4                                                                              0.3                                                                              0.044                                                                            0.008                                                                             0.001                                                                            Comparison                                     17 0.15                                                                             0.33                                                                             1.60                                                                             0.010              Example                                        18 0.15                                                                             0.33                                                                             1.60                                                                             0.010              Example                                        19 0.15                                                                             0.33                                                                             1.60                                                                             0.010              Comparison                                     __________________________________________________________________________

                  TABLE 1 (B)                                                     ______________________________________                                                                       Maxi-                                               Yield    Tensile  Martensitic                                                                           mum                                                 Strength Strength Phase Ratio                                                                           Load                                           No.  Kgf/mm.sup.2                                                                           Kgf/mm.sup.2                                                                           %       Kgf    Remarks                                 ______________________________________                                         1   118      125      100     1202   Example                                  2   130      135      100     1297   Example                                  3   122      162      100     1558   Example                                  4   142      154      100     1483   Example                                  5   145      168      100     1618   Example                                  6   139      153      100     1475   Example                                  7   141      158      100     1522   Example                                  8   137      147       76     1413   Example                                  9   127      132       80     1272   Example                                 10   138      152      100     1461   Example                                 11   143      154      100     1481   Example                                 12   151      159      100     1532   Example                                 13   115      125       65     1205   Example                                 14   134      140      100     1346   Example                                 15   107      118       58     1098   Comparison                              16    95      105       52     1011   Comparison                              17   105      131       70     1261   Example                                 18    85      122       62     1174   Example                                 19   109      115       50     1087   Comparison                              ______________________________________                                    

What is claimed is:
 1. An electric-resistance-welded steel pipe withhigh strength, consisting of C, Mn, Si, Nb and Fe in the followingamounts: 0.05 to 0.3 wt. % C, 0.6 to 2 wt. % Mn, 0.03 to 0.7 wt. % Si,0.01 to 0.05 wt. % Nb, and the balance being Fe and inevitableimpurities, said steel pipe having a martensitic phase in an amount of60 to 100% and having a tensile strength of 120 to 170 kgf/mm².
 2. Theelectric-resistance-welded steel pipe of claim 1, consisting of 0.20 wt.% C, 0.40 wt. % Si, 1.76 wt. % Mn and 0.015 wt. % Nb.
 3. Theelectric-resistance-welded steel pipe of claim 1, consisting of 0.15 wt.% C, 0.33 wt. % Si, 1.60 wt. % Mn and 0.010 wt. % Nb.
 4. Theelectric-resistant-weld steel pipe of claim 1, wherein the Nb is in anamount of 0.01 to 0.015 wt. %.
 5. The electric-resistant-welded steelpipe of claim 1, produced bycontinuously annealing a steel sheet bysoaking the steel sheet at a temperature of 800° to 900° C., quenchingthe soaked steel sheet at a rate of 800° to 1500° C./second andheat-treating at a temperature of 100° to 300° C.; and forming thecontinuously annealed steel sheet into an electric-resistance-weldedsteel pipe.